The present invention relates to a detector for detecting the position of forks in industrial vehicles such as forklifts. More specifically, the present invention pertains to a technology for detecting abnormalities in wires that transmit position detection signals.
In industrial vehicles such as forklifts, a pair of inner masts is supported in a pair of outer masts. The inner masts slide up and down in the outer masts and a fork moves with the inner masts. In such forklifts, various controls are performed in accordance with the position of the fork. For example, the fork may be automatically stopped at a position set in advance. The rear axle is pivotally supported for improved driving performance. When the fork is raised high and the center of gravity of the vehicle is high, the pivoting movement of the rear axle is restricted to improve stability.
Generally, a reel-type sensor is used to continuously detect the height of fork. The reel-type sensor detects the rotation of a reel, which winds and unwinds a wire in accordance with the movement of the fork, to determine the fork height. Alternatively, switch-type sensors may be used to intermittently detect the height of the fork. A plurality of switch-type detectors is installed on the outer masts, spaced at a certain distance from one another. A dog is attached to the inner masts to actuate the switches. The number of detectors turned on in accordance with the height of the inner masts changes. Accordingly, the height of forklift is intermittently detected based on the on-off state of the detectors.
Whether to continuously detect or intermittently detect the fork height is determined by the kind of control performed. For example, detecting only three height zones, that is, a lower zone, a middle zone, and a high zone may be sufficient. In this case, an inexpensive switch-type sensor is preferred over a more costly reel-type sensor.
When switch-type detectors are used, a plurality of switch-type detectors are arranged vertically on the outer masts with a predetermined distance from one another. Wires from each switch-type detector extend along the outer masts. However, when the wires slacken by the movement of the inner masts, the wires may be caught and cut by the inner masts. The cut wires may be short-circuited by touching the masts or the vehicle body. This is likely to cause abnormalities in the electrical system.
Such abnormalities must be detected quickly. However, in prior art apparatuses that detect the height in accordance with the on-off state of the switch-type detectors, abnormalities such as a cut line and a short circuit cannot be detected. Therefore, as a safety measure, the signal input to the controller when the fork is at the highest position is made to be the same as that when there is a cut wire. In this way, when a wire is cut, the control used when the fork is at the highest position is performed. When the fork is at the highest position, the center of the gravity of the vehicle is high, which lowers the stability of the vehicle. Thus, if a wire is cut, the control procedure for stabilizing the vehicle is performed at all times.
This safety measure does not detect cut wires and is not a fully satisfactory solution. If an operator does not notice the abnormality, proper control in accordance with the fork height is not performed, which lowers the performance of the forklift.
When the number of wires extending from the switch-type detector is large, a wire is more likely to be cut. Further, a controller needs many input terminals. Therefore, it is desirable to reduce the number of wires employed with the switch-type detectors.